<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
         .box{
            width: 600px;
            height: 600px;
            background: red;
        }
        @media screen and (max-width: 600px) {
           .box {
               width: 600px;
               height: 600px;
               background: blue;
           }
    }
       
    </style>
</head>
<body>
    <div class="box">

    </div>
</body>
</html>